DFT Study of Polyaniline and Metal Composites as Nonprecious Metal

Article pubs.acs.org/JPCC

DFT Study of Polyaniline and Metal Composites as Nonprecious Metal Catalysts for Oxygen Reduction in Fuel Cells Xin Chen,† Shaorui Sun,*,† Xiayan Wang,† Fan Li,† and Dingguo Xia*,‡ †

College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China College of Engineering, Peking University, Beijing 100871, China

‡

ABSTRACT: A theoretical study on the oxygen reduction mechanism catalyzed by metal−polyaniline is investigated in detail by means of density functional theory. In the oxygen reduction process, we find that −OH, not H2O2, is the reaction intermediate. The catalytic activities for the studied models decrease in the sequence CoFe−PANI > Fe−PANI (FeFe−PANI) > Co−PANI. This is due to a synergistic effect between heterogeneous metal atoms in CoFe−PANI, which facilitates additional electron donation from the active sites to the adsorbed oxygen reduction intermediates. The doping with cobalt may also decrease the HOMO−LUMO gap in CoFe−PANI, making it more active.

1. INTRODUCTION Polymer electrolyte fuel cells (PEFCs) are regarded as one of the most promising candidates for stationary and mobile power generation due to their large energy yield and the low environmental impact of hydrogen oxidation. However, the cost of platinum (Pt) or Pt alloys serving as catalysts is prohibitive in PEFC in light of the high loading levels that are required for the oxygen reduction reaction (ORR).1 Consequently, the development of nonprecious metal catalysts (NPMCs) with high ORR activity and durability is a major focus of PEFC research.2−5 Recently, Fe/Co−Nx materials have revealed their great potential as one type of NPMC. Although impressive advances have been recently achieved, the understanding of the origin of the catalytic activity in Fe/Co−Nx materials is far from complete. For Fe/Co−Nx-based NPMCs, there generally are a number of large challenges encountered in determining the structure of the active sites. First, metal atoms may either be at the center of the active sites, or they may merely serve to catalyze the formation of the active sites from carbon and nitrogen.6−9 Second, the number of active sites is very sparse relative to the number of sites in the whole system, which makes their structure difficult to detect. Third, for Fe/Co−Nx materials, different kinds of active sites may contribute to the overall catalytic activity, even in one single type of material.10 Despite the many debates that exist on these issues, the following consensuses have been reached regarding Fe/Co−Nx materials:11 (i) the simultaneous presence of a transition metal and nitrogen plays an important role in the formation of active sites of the catalysts; (ii) nitrogen is a crucial component in the active sites; and (iii) the catalysts prepared with Co- or Fecontaining precursors are more active than those that use other transition metal (nonprecious metal) precursors. © 2012 American Chemical Society

Because the active site structure of Fe/Co−Nx materials after heat treatment is not yet clearly understood, it has proven to be difficult to determine the oxygen reduction mechanism. Some electroconductive polymers have been used as ligands to synthesize a new class of both pyrolyzed and nonpyrolyzed metal-doped polymer complex catalysts, owing to the high porosity, conductivity, and large effective area of such polymers.5,12−20 It has also been reported that nonpyrolyzed cobalt−polypyrrole has a high ORR activity, without showing any noticeable loss of performance during operation of the PEFC.5 Polyaniline is another kind of electroconductive polymer. Although a Fe−PANI catalyst prepared by heat treatment is described in the literature, it remains of interest to study nonpyrolyzed Fe/Co−PANI as an ORR catalyst for several reasons: first, the metal−PANI ligand structure might be determined so that the ORR mechanism could be simulated, which may lead to the discovery of the catalytic process in other Fe/Co−Nx materials; second, the synergistic effect between double metals (Fe and Co), which improves the ORR catalytic activity, could be studied with such a model, and third, the Fe/ Co−PANI may potentially be a powerful catalyst for ORRs. To shed light on the origin of their catalytic activity and the nature of their active sites, four kinds of metal−polyaniline molecular fragments were constructed, namely, Co−PANI, Fe− PANI, FeFe−PANI, and CoFe−PANI, and their ORR mechanism was investigated by way of density functional theory (DFT). The results indicate that the doping of monometal−polyaniline with another transition metal might enhance its catalytic activity somewhat. Received: July 16, 2012 Revised: October 4, 2012 Published: October 8, 2012 22737

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Figure 1. Schematic representation of metal−polyaniline molecular fragments: (a) Co−PANI, (b) Fe−PANI, (c) FeFe−PANI, and (d) CoFe− PANI.

2. COMPUTATIONAL METHODS DFT calculations were carried out using the Amsterdam Density Functional program package (ADF, version 2010.01).21−23 All oxygen reduction steps in the system were performed using the Becke (exchange) and the Lee−Yang− Parr (correlation)24 functional (BLYP). The metal atoms, cobalt and iron, were modeled with a triple-ζ polarized (TZP) slater-type basis set, whereas other atoms were based on double-ζ polarized (DZP) sets. This method of mixing basis sets of different sizes is consistent with Ribas and Dragonetti’s works,25,26 which used TZP basis set for metal atom and DZP for nonmetallic atoms. Furthermore, Michalak’s work also confirmed that the basis set effect for such systems is clearly very small.27 The inner core orbitals, 1s for C, N, and O and 1s−3p for Co and Fe, were kept frozen. All atoms were allowed to relax without constraint in the geometry optimizations. For all stationary states, spin multiplicity was allowed to relax: possible geometries with varying spin states were carefully checked, and the ground state was determined to be the one with the lowest electronic energy. Furthermore, the atomic charges were obtained via multipole derived charge analysis (MDC-q),28 which yields charges that reproduce (by construction) both the atomic and the molecular multipoles. The chemical potential (the free energy per H atom) for the reaction (H+ + e−) can be derived from that of 1/2 H2 in the gaseous phase by use of the standard hydrogen electrode model.29−31 Therefore, under standard conditions (U = 0, pH 0, p = 1 bar, T = 298 K), the free-energy difference in the reaction of type *AH → A + H+ + e− may be calculated from the free energy in the reaction given by *AH → A + 1/2H2. The change in free energy for a reaction is calculated using ΔG = ΔE + ΔZPE − TΔS, where ΔE is the reaction energy, ΔZPE is the difference in zero point energy, T is temperature, and ΔS is the change in entropy. In our work, the values of ΔE and ΔZPE were calculated via DFT, whereas the value for entropy is taken from a chemical database.32 Four metal−polyaniline molecular fragments were constructed to simulate the oxygen reduction mechanism of a catalyst (Figure 1). In each model, the metal atom is directly bonded to two polyaniline chains, thus generating a metal−Nx active site. The structural models of Co− and Fe−PANI are based on ref 16. The other two structural models, FeFe− and CoFe−PANI, are studied to estimate the synergistic effect

between the two metal atoms, as evidenced by their higher catalytic performance.

3. RESULTS AND DISCUSSION 3.1. Stability Study. To evaluate the stability of the selected model structures, we calculated the energy change (ΔE) for the reactions between hydrated metal ions and polyaniline, as shown below. As is well-known, Co(II) and Fe(II) ions are both coordinated with six water molecules in solution.32 For an exothermic reaction (a negative heat value), the calculations suggest that the metal−polyaniline structure is more energy favorable than the structure involving hydrated metal ions. The calculated results are listed in Table 1. 2(H − PANI4) + [Co(H 2O)6 ]2 + → PANI4 − Co − PANI4 + 2[H3O+(H 2O)2 ]

(1)

2(H − PANI4) + [Fe(H 2O)6 ]2 + → PANI4 − Fe − PANI4 + 2[H3O+(H 2O)2 ]

(2)

2(2H − PANI4) + 2[Fe(H 2O)6 ]2 + → PANI4 − FeFe − PANI4 + 4[H3O+(H 2O)2 ]

(3)

2(2H − PANI4) + [Co(H 2O)6 ]2 + + [Fe(H 2O)6 ]2 + → PANI4 − CoFe − PANI4 + 4[H3O+(H 2O)2 ]

(4)

Table 1. Calculated Reaction Energies structure

system

ΔE (eV)

Co−PANI Fe−PANI FeFe−PANI CoFe−PANI

PANI4−Co−PANI4 PANI4−Fe−PANI4 PANI4−FeFe−PANI4 PANI4−CoFe−PANI4

−1.42 −0.91 −1.78 −2.11

From Table 1, it can be seen that each reaction has a negative ΔE value, indicating that all calculated reactions are exothermic. In other words, the selected metal−polyaniline structures are more stable than their hydrated metal ions. 3.2. Oxygen Adsorption. The generally sluggish ORR kinetics are due to the high strength of the O−O bond; the function of the catalyst is to facilitate the dissociation of that 22738

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the end-on configuration because of an increased d−π* orbital overlap between metal and O2 molecule.34 The calculated bond lengths of O2 molecules in states A1, C1, and D1 are 1.416, 1.444, and 1.442 Å, respectively. In the four metal−PANI models, the Fe−, FeFe−, and CoFe−PANI perform the O2 bond elongations to nearly the same degree, all of them longer than that of Co−PANI. The free-energy change for O2 in CoFe−PANI is −0.75 eV, which is the lowest of the four models. (The O2 molecule is adsorbed on the surface of Fe atom because the highest occupied molecular orbital is mainly composed of Fe atom in CoFe−PANI’s structure.) The data above indicate that the first electron-transfer step on Co− and Fe−PANI may be somewhat difficult due to the stronger adsorption of O2.26 In other words, the first oxygen reduction step on CoFe−PANI is more energy favorable than in each one of the other models. 3.3. Potential Energy Surface Profile for ORR. Potential energy surface profile is a useful method to evaluate the mechanism of ORR.35 In the present study, the following oxygen reduction steps were modeled.

bond. Therefore, the catalyst’s ability to interact with oxygen is essential to its catalytic activity. If the interaction between the catalyst and oxygen (the metal−O interaction) is very weak, then the coverage of oxygen may be too low to allow for a high reaction rate. If oxygen bonds too strongly with the catalyst, then it will poison the surface and reduce its activity. In general, there are two dioxygen adsorption modes: end-on and side-on. In this work, for each of the four metal− polyaniline models, the two adsorption modes are both calculated and their structures are fully optimized (Figure 2).

Figure 2. Structure of dioxygen adsorbed on Fe−PANI with (a) endon mode and (b) side-on mode. The blue-gray circle is an iron atom, the blue circles are nitrogen atoms, the gray circles are carbon atoms, the red circles are oxygen atoms, and the small light circles are hydrogen atoms.

The calculation results suggest that the electronic energy for the side-on adsorption is lower than that for the end-on adsorption (for example, for Fe−PANI, the side-on adsorption energy is lower by ∼0.96 eV than that for the end-on adsorption), which indicates that the side-on adsorption is more stable than the end-on type. As a result, the side-on adsorption modes are adopted. To simplify the discussion, the initial states of Co−, Fe−, FeFe−, and CoFe−PANI are specified as A0, B0, C0, and D0, respectively. When O2 is adsorbed on the active site, the corresponding states are named as A1, B1, C1, and D1. For Fe− PANI, one finds that O2 is adsorbed in a configuration that is nearly side-on. (See Table 2.) This is due to a steric hindrance

molecule

R(Me−Oa)

R(Me−Ob)

R(Oa−Ob)

1.830 1.836 1.835 1.776

1.885 1.842 1.844 1.938

1.416 1.441 1.444 1.442

(5)

A 0 − O2 + H+ + e−(U) → A 0 − OOH(A 2)

(6)

A 0 − OOH + H+ + e−(U) → A 0 − OH − OH(A3) (7)

A 0 − OH − OH + H+ + e−(U) → A 0 − OH + H 2O(A4)

(8)

A 0 − OH + H+ + e−(U) → A 0 − OH 2(A 5)

A 0 − OH 2 → A 0 + H 2O

(9) (10)

+

−

If the free energy of A0 + O2 + 4(H + e ), B0 + O2 + 4(H+ +e−), C0 + O2 + 4(H+ + e−), and D0 + O2 + 4(H+ + e−) is specified as 0 eV, then the relative free energies of A1(B1, C1, D1) + 4(H+ + e−), A2(B2, C2, D2) + 3(H+ + e−), A3(B3, C3, D3) + 2(H+ + e−), A4(B4, C4, D4) + H2O + (H+ + e−), and A5(B5, C5, D5) + H2O are illustrated in the energy profile shown in Figure 3.

Table 2. Calculated Equilibrium Internuclear Distances, R (Å), for Co−, Fe−, FeFe−, and CoFe−PANI Co−PANI Fe−PANI FeFe−PANI CoFe−PANI

A 0 + O2 → A 0 − O2 (A1)

effect that is a result of the geometry of Fe−PANI, where one of the aniline rings blocks the O2 from freely approaching the Fe atom at the center of the aniline rings and thus prevents O2 from forming a complete side-on configuration. The situation is nearly the same in the other three models. For Fe−PANI, the change in the Gibbs free energy (ΔG) associated with O2 adsorption is about −1.37 eV, and the shortest distance between the Co atom and one O atom is 1.836 Å. When an O2 molecule is adsorbed on the Fe−PANI surface, the O−O bond length is elongated from an equilibrium value of 1.245 to 1.441 Å, indicating that the molecular oxygen has been activated by the catalyst. This value is significantly higher than the elongations obtained for O2 adsorption on metallophthalocyanine or metallooporphyrin, where O2 is adsorbed in the end-on configuration.33 This is because the strength of the O−O bond for the side-on configuration is generally weaker than that for

Figure 3. Potential energy surface profiles for O2 reduction in the four reaction systems.

For the first O2 reduction step (eq 6), the values of ΔG are 0.26 and 0.13 eV for Co− and Fe−PANI, respectively (Figure 3), indicating that they are both endergonic. For FeFe− and CoFe−PANI, the ΔG values are −1.24 and −2.13 eV, which indicates that CoFe−PANI is the most energy favorable arrangement. 22739

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levels should decrease in the sequence CoFe−PANI> Fe− PANI (FeFe−PANI) > Co−PANI. The adsorption energies for −OH and −OOH on the four metal−PANIs are listed in Table 3, where it can be seen that CoFe−PANI presents the strongest adsorption for the two oxygen reduced intermediates, which manifests as an −O adsorption. Table 4 shows the net atomic charges for the entire O2 reduction intermediates as produced by MDC-q population analyses. Among the four models, the intermediate species catalyzed by CoFe−PANI accumulate the most negative charges. This clearly implies that within a certain range the large net charge of the adsorbed intermediates could induce a high adsorption strength, and the high adsorption strength in turn could result in a high level of catalytic activity. Experimental results indicate that the strong −OH adsorption on Pt may cause an overpotential,39 because the −OH might cover the Pt surface and block the adsorption of molecular oxygen in the subsequent step. In the final oxygen reduction step (eq 9), which is catalyzed by a metal−PANI, additional externally supplied energy would be needed to complete the −OH reduction process, especially for CoFe− PANI (see Figure 3). Therefore, although the CoFe−PANI has the highest catalytic activity among the four catalysts, the possibility of an overpotential caused by strong −OH adsorption may weaken its catalytic behavior. 3.5. Synergistic Effect. As discussed above, when the electron-withdrawing group (−OO, −OOH, −O, or −OH) is adsorbed on the active site, an electron flows from the catalyst molecule to the group (see Table 4). For Fe−PANI, the maximum change in charge (absolute value) for iron in the ORR is ∼21%, as illustrated in Figure 5 a, and for each nitrogen (in one of the aniline chains) the change is Co−PANI. This is the result of a synergistic effect between the heterogeneous metal atoms in CoFe−PANI, which allows it to donate additional electrons from the active site to the adsorbed oxygen reduction intermediates. The doping of cobalt could also decrease the HOMO−LUMO gap in CoFe−PANI and thus make it more active.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 10 67396158. Fax: +86 10 67391983. E-mail: sunsr@ bjut.edu.cn (S.S.), [email protected] (D.X.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was performed with the financial support from the Fundamental Science Research Foundation of Beijing University of Technology (X4005012201101), the major program of Beijing Municipal Natural Science Foundation (no. 20110001), and National Natural Science Foundation of China (no. 11179001).

Figure 6. Energy levels in CoFe−PANI and FeFe−PANI (α = spin-up orbital; β = spin-down orbital).

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where it appears that the HOMO energy levels for both spin-up and spin-down orbitals in CoFe−PANI are all higher than the corresponding levels in FeFe−PANI. This suggests that the doped cobalt might not only serve to decrease the HLG but also to enhance the HOMO level and thus facilitate electron transfer. On the basis of the HLG values, we conclude that Co−PANI is the most inactive catalyst among the four models. The ordering of the sequence of catalytic activity is confirmed once more and is consistent with the result obtained by the adsorption analysis for the oxygen reduction intermediates. In CoFe−PANI, fluctuations in the d-orbital configuration are induced by the presence of double metals (Fe and Co), which might further enhance the catalytic activity. This idea

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